1. Field of the Invention
The present invention relates generally to systems and methods of tuning power amplifier outputs and more particularly, to systems and methods for tuning a single output of a differential power amplifier.
2. Description of the Related Art
Transceiver power efficiency is greatly dependant on the efficiency of the transmitter power amplifier (PA). An efficient PA converts as much of the bias current to RF output as possible. PA efficiency is especially important in portable transceiver systems that rely on a portable power source (e.g., battery) or other transmitters that have a limited power supply. Many portable transmitters are manufactured as highly integrated circuits (i.e., transmitter on a chip) so as to exploit the power efficiencies of integrated circuit design.
Some of the potential transmitter inefficiencies can be eliminated or significantly reduced in the design of the integrated power amplifier components. However, an integrated PA must still be connected to an antenna, impedance matching network, balancing circuits and other components that are external to the transmitter on a chip. These external connections result in at least some parasitic capacitance in the output of the PA that cannot be eliminated in or efficiently compensated for on the chip.
FIG. 1 shows a block diagram of a typical prior art transceiver 100. The transceiver 100 includes an integrated transmitter 104 that includes a differential power amplifier 110. The transceiver 100 also includes a front-end circuit 102. The front-end circuit 102 includes a balun 114. The differential PA 110 has a positive potential output 110p and negative potential output 110n. The outputs 110p 110n of the PA 110 are coupled to the corresponding inputs 114p, 114n of the balun 114. The output 114A of the balun 114 is coupled to an antenna 120.
The balun 114 is a balanced signal to unbalanced signal converter circuit that converts the balanced input signals 110n, 110p to an unbalanced or single pole output signal 114A, such as may be coupled to a single pole antenna 120 to output a transmitter output signal. Each of the components in the front-end circuit 102 (e.g., the balun 114, the antenna 120 and the interconnecting wires) has some level of parasitic capacitance that can load or otherwise degrade the efficiency of the PA 110.
Prior art approaches to manage parasitic output capacitance of the PA 110 include placing one or more inductors in the front-end circuit 102 (e.g., inductors 124p, 124n) or one or more inductors (e.g., inductors 126p, 126n) in the transmitter 104 (i.e., within the same chip as the transmitter 104). However, an effective integrated inductor consumes too much precious space within the transmitter chip 104. Therefore placing an inductor within the transmitter 104 is an inefficient use of the available space on the chip 104 and therefore significantly increases the cost.
Placing an inductor in the front-end circuit 102 can require the inductor to be individually tuned to fit the precise needs of the output of the PA 110. Further, the inductive component variations between manufacturers or even slight variations in the installation of the inductors in the front-end circuit can require still additional tuning.
Maximum power transfer occurs when the input impedance of the balun 114 is the complex conjugate of the PA output impedance ZPA out. Nominally, the PA output impedance is designed to be entirely resistive RPA out, but ultimately at least some on-chip parasitic capacitance XPA out cannot be avoided. Therefore, for maximum power transfer the balun input impedance must have an equal resistive part Rbalun with an inductive reactance Xbalun as shown by the following relationships:ZPAout=RPAout+XPAout Zbalun=Rbalun+Xbalun RPAout=Rbalun XPAout=−Xbalun 
As shown in the above relationships, the balun input impedance Zbalun, must appear to be inductive to the PA 110 while maintaining balun functionality and differential balance in the PA 110 output.
FIG. 2 shows a block diagram of another prior art power amplifier output circuit 200. The differential power amplifier outputs PA+ 204 and PA− 206 are input to a balun 202. The DC bias for the power amplifier is routed through two DC bias networks 207A, 207B. DC bias for the positive PA output 204 is supplied through DC bias network 207A while DC bias for the negative PA output 206 is supplied through DC bias network 207B. DC bias network 207A includes a quarter wavelength (λ/4) stub 210A and DC bias network 207B includes a quarter wavelength (λ/4) stub 210B. A quarter wavelength stub acts as an open to RF, therefore, the quarter wavelength stubs 210A, 210B protect the DC bias supplies 212A, 212B by blocking RF from being fed back through the DC bias networks 207A, 207B from the PA outputs 204, 206.
Another prior art approach was to apply varying lengths of tuning stubs 214A, 214B to the respective PA outputs 204, 206. By way of example, a first tuning stub 214A could be connected to the positive PA output 204 and a second tuning stub 214B could be connected to the negative PA output 206. However, these two tuning stubs 214A, 214B must have precisely the same dimensions, with very close tolerances. If the two tuning stubs 214A, 214B were not precisely the same dimensions, they could create additional imbalances on the PA output. The length of the tuning stubs 214A, 214B is adjusted to provide the desired tuning function (i.e., to compensate for parasitic capacitance presenting the PA outputs 204, 206).
Still another approach was to put more than one stub on each of the balanced PA outputs 204, 206. However, each of these stubs can provide yet additional capacitance and additional variation that could cause still more imbalances that must be corrected.
What is needed is a system and method to make a device in the front-end circuit, such as a balun, that appears to be inductive to the output of the PA and that will also compensate for the parasitic capacitance of the PA output connections. In addition, the compensation must maintain balance of the PA outputs while also protecting the DC bias source from any RF output from the PA.